Messages in this thread | | | From | bsegall@google ... | Subject | Re: [PATCH 1/2] sched: Defend cfs and rt bandwidth quota against overflow | Date | Wed, 22 Apr 2020 11:44:30 -0700 |
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changhuaixin <changhuaixin@linux.alibaba.com> writes:
>> 在 2020年4月21日,上午1:50,bsegall@google.com 写道: >> >> Huaixin Chang <changhuaixin@linux.alibaba.com> writes: >> >>> Kernel limitation on cpu.cfs_quota_us is insufficient. Some large >>> numbers might cause overflow in to_ratio() calculation and produce >>> unexpected results. >>> >>> For example, if we make two cpu cgroups and then write a reasonable >>> value and a large value into child's and parent's cpu.cfs_quota_us. This >>> will cause a write error. >>> >>> cd /sys/fs/cgroup/cpu >>> mkdir parent; mkdir parent/child >>> echo 8000 > parent/child/cpu.cfs_quota_us >>> # 17592186044416 is (1UL << 44) >>> echo 17592186044416 > parent/cpu.cfs_quota_us >>> >>> In this case, quota will overflow and thus fail the __cfs_schedulable >>> check. Similar overflow also affects rt bandwidth. >> >> More to the point is that I think doing >> >> echo 17592186044416 > parent/cpu.cfs_quota_us >> echo 8000 > parent/child/cpu.cfs_quota_us >> >> will only fail on the second write, while with this patch it will fail >> on the first, which should be more understandable. >> >> >> to_ratio could be altered to avoid unnecessary internal overflow, but >> min_cfs_quota_period is less than 1<<BW_SHIFT, so a cutoff would still >> be needed. >> > > Yes, I will rewrite commit log in the following patch. > >> Also tg_rt_schedulable sums a bunch of to_ratio(), and doesn't check for >> overflow on that sum, so if we consider preventing weirdness around >> schedulable checks and max quotas relevant we should probably fix that too. >> > > It seems to me that check for overflow on sum of to_ratio(rt_period, rt_runtime) > is not necessary. As to_ratio() of a rt group is bounded by global_rt_period() > and global_rt_runtime() due to the checks in tg_rt_schedulable(). And > global_rt_runtime() is not allowed to be greater than global_rt_period() thanks > to sched_rt_global_validate(). Thus, to_ratio() of a rt group will not exceed > BW_UNIT, sum of which is unlikely to overflow then. Checks against rt_runtime > overflow during to_ratio is still needed. > > Is that correct?
Good point, that's probably not a problem then.
> >>> >>> Signed-off-by: Huaixin Chang <changhuaixin@linux.alibaba.com> >>> --- >>> kernel/sched/core.c | 8 ++++++++ >>> kernel/sched/rt.c | 9 +++++++++ >>> kernel/sched/sched.h | 2 ++ >>> 3 files changed, 19 insertions(+) >>> >>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c >>> index 3a61a3b8eaa9..f0a74e35c3f0 100644 >>> --- a/kernel/sched/core.c >>> +++ b/kernel/sched/core.c >>> @@ -7390,6 +7390,8 @@ static DEFINE_MUTEX(cfs_constraints_mutex); >>> >>> const u64 max_cfs_quota_period = 1 * NSEC_PER_SEC; /* 1s */ >>> static const u64 min_cfs_quota_period = 1 * NSEC_PER_MSEC; /* 1ms */ >>> +/* More than 203 days if BW_SHIFT equals 20. */ >>> +static const u64 max_cfs_runtime = MAX_BW_USEC * NSEC_PER_USEC; >>> >>> static int __cfs_schedulable(struct task_group *tg, u64 period, u64 runtime); >>> >>> @@ -7417,6 +7419,12 @@ static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota) >>> if (period > max_cfs_quota_period) >>> return -EINVAL; >>> >>> + /* >>> + * Bound quota to defend quota against overflow during bandwidth shift. >>> + */ >>> + if (quota != RUNTIME_INF && quota > max_cfs_runtime) >>> + return -EINVAL; >>> + >>> /* >>> * Prevent race between setting of cfs_rq->runtime_enabled and >>> * unthrottle_offline_cfs_rqs(). >>> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c >>> index df11d88c9895..f5eea19d68c4 100644 >>> --- a/kernel/sched/rt.c >>> +++ b/kernel/sched/rt.c >>> @@ -2569,6 +2569,9 @@ static int __rt_schedulable(struct task_group *tg, u64 period, u64 runtime) >>> return ret; >>> } >>> >>> +/* More than 203 days if BW_SHIFT equals 20. */ >>> +static const u64 max_rt_runtime = MAX_BW_USEC * NSEC_PER_USEC; >> >> It looks to me like __rt_schedulable doesn't divide by NSEC_PER_USEC, so >> to_ratio is operating on nsec, and the limit is in nsec, and MAX_BW_USEC >> should probably not be named USEC then as well. > > Yes, the limit for rt_runtime is in nsec. This should be changed. > >> >>> + >>> static int tg_set_rt_bandwidth(struct task_group *tg, >>> u64 rt_period, u64 rt_runtime) >>> { >>> @@ -2585,6 +2588,12 @@ static int tg_set_rt_bandwidth(struct task_group *tg, >>> if (rt_period == 0) >>> return -EINVAL; >>> >>> + /* >>> + * Bound quota to defend quota against overflow during bandwidth shift. >>> + */ >>> + if (rt_runtime != RUNTIME_INF && rt_runtime > max_rt_runtime) >>> + return -EINVAL; >>> + >>> mutex_lock(&rt_constraints_mutex); >>> err = __rt_schedulable(tg, rt_period, rt_runtime); >>> if (err) >>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h >>> index db3a57675ccf..6f6b7f545557 100644 >>> --- a/kernel/sched/sched.h >>> +++ b/kernel/sched/sched.h >>> @@ -1918,6 +1918,8 @@ extern void init_dl_inactive_task_timer(struct sched_dl_entity *dl_se); >>> #define BW_SHIFT 20 >>> #define BW_UNIT (1 << BW_SHIFT) >>> #define RATIO_SHIFT 8 >>> +#define MAX_BW_BITS (64 - BW_SHIFT) >>> +#define MAX_BW_USEC ((1UL << MAX_BW_BITS) - 1) >>> unsigned long to_ratio(u64 period, u64 runtime); >>> >>> extern void init_entity_runnable_average(struct sched_entity *se);
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